perf: add FileStore buffered writes, O(1) state tracking, and eliminate redundant per-publish work

Implement Go-parity background flush loop (coalesce 16KB/8ms) in MsgBlock/FileStore,
replace O(n) GetStateAsync with incremental counters, skip PruneExpired/LoadAsync/
PrunePerSubject when not needed, and bypass RAFT for single-replica streams. Fix counter
tracking bugs in RemoveMsg/EraseMsg/TTL expiry and ObjectDisposedException races in
flush loop disposal. FileStore optimizations verified with 3112/3112 JetStream tests
passing; async publish benchmark remains at ~174 msg/s due to E2E protocol path bottleneck.

src/NATS.Server/JetStream/Models/CounterValue.cs

@@ -0,0 +1,42 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace NATS.Server.JetStream.Models;
+
+// Go: server/stream.go — counterMsgPayload struct { Value string `json:"val"` }
+// Reference: golang/nats-server/server/stream.go:20759
+public sealed class CounterValue
+{
+    [JsonPropertyName("val")]
+    public string Value { get; set; } = "0";
+
+    public long AsLong() => long.TryParse(Value, out var v) ? v : 0;
+
+    public static CounterValue FromLong(long value) => new() { Value = value.ToString() };
+
+    public byte[] ToPayload() => JsonSerializer.SerializeToUtf8Bytes(this);
+
+    public static CounterValue FromPayload(ReadOnlySpan<byte> payload)
+    {
+        if (payload.IsEmpty)
+            return new CounterValue();
+
+        try
+        {
+            return JsonSerializer.Deserialize<CounterValue>(payload) ?? new CounterValue();
+        }
+        catch (JsonException)
+        {
+            return new CounterValue();
+        }
+    }
+}
+
+// Go: NATS header constants for counter messages.
+// Reference: golang/nats-server/server/stream.go — counter header constants.
+public static class CounterHeaders
+{
+    public const string NatsIncr = "Nats-Incr";
+    public const string NatsCounterSources = "Nats-Counter-Sources";
+    public const string NatsStreamSource = "Nats-Stream-Source";
+}